e-4WD?

Tried reading through the last link a bit: rambling, mindless, self-congratulatory drivel.

 

HSD always had two motors. The AWD will put a third one on the rear axle.

 

It is entirely possible that they mean a plurality of motors at the rear axle.

Because with a single motor, you would need a differential, which has its own weight/space/maintenance requirements. With a plurality of motors serving each individual half-shaft, not only do you no longer need a differential, you now also have the option of active torque-biasing steering and traction control.

 

Toyota Showcases The New RAV4 Hybrid At Frankfurt

Available in both front and all-wheel drive variants, the latter comes equipped with a second, 50 kW high-voltage, rear mounted electric motor that offers increased traction and a 1,650 Kg towing capacity – without the added weight and complexity of a central drive shaft. The engine is capable of operating independently from the hybrid system front’s electric motor, driving the rear wheels alone. An E-Four system smoothly switches theRAV4 to all-wheel drive on slippery road surfaces and from stationary starts, thus maximizing traction.

 

Not likely.
Honda uses two motors on the rear axle of their new performance AWD hybrid. It's on the RLX.
Toyota has only used a single motor at the rear since the RX hybrid came out. They could do two motors, but that will cost more. It might appear on a Lexus power hybrid, but not on an affordable people mover.

 

I'm pretty sure it's not as much about cost, but more of an issue of application.

The primary characteristic in favor of a single motor + diff system is that this system can direct nearly 100% of the total available torque available on that axle to just one wheel of the axle by braking the opposing wheel. This is obviously important in a vehicle that might be expected to exhibit some semblance of off-road capability, such as the RX.

A non-diff, individual motor system can only ever direct 50% of the torque to each wheel on an axle. This is perfectly fine for a primarily road-going car, such as the RLX.

 

Yes, we will have to see if Toyota provides any official Prius e-4WD information. Interesting that in 2012 AutoFoundry reported 4WD speculation referencing the same 37 mph limitation as quoted in today's AutoExpress report from the Frankfurt Auto Show.

 

Read the fine print, the Highlander and RX are not designed nor intended for off road use. Those hybrids with some off road chops, the Escape and GM two-modes, used a mechanical AWD or 4WD system.
Cost or packaging are the likely reasons Toyota went with one motor for the rear. We haven't seen an eAWD on a car yet though.

 

The press release may only be speaking of the MG2. The MG1 may not have had any reduction since it is already a smaller unit that acts the part of the starter, alternator, and helps control the variable speeds of the transmission.

 

Locking one wheel where there is a differential, doesn't change the torque available at the other wheel. If the input RPM from the motor is unchanged, it doubles the rpm at which the unlocked wheel spins

 

Press releases are generally written by people with journalism or marketing backgrounds.
They often try to simplify what the engineers tell them in the hope of making it more accessible to non technical people and end up putting out stuff that isn't quite right

 

Those vehicles "are not intended for off-road use" primarily because they don't have the frame strength required. I don't think the drive system has much to do with it. The Prius, if so equipped, will also not have the frame strength. But if you really want to, go ahead and run your Prius "off-road". More spare parts in the junkyard for the rest of us.

The Highlander Hybrid uses a single rear motor (for a total of three), on the input to a diff. It would be "better" to use two motors on the rear wheels, one each, so you could control the wheel torque better, but that would be, as already posted, rather expensive.

And just to underline the real reason, BOTH the non-Hybrid and Hybrid Highlander "are not intended for off-road use". Whatever "off-road use" actually means. Is a trail a road? It is if people drive on it. But I digress.

 

Yes, it does. The torque available at the wheels is the same for a conventional differential. This is why having one wheel on ice and one on pavement won't help you to get unstuck - the wheel on the pavement can't absorb any more torque than the wheel on the ice.

If you apply the brake to the wheel on the ice, more torque can be provided to the wheel on the pavement because the torque going the other way is increased.

 

Actually, energyandair is right. I had a brain fart in terms of remembering how the torque distribution of an open diff works. When you lock one output of an open diff, the rpm of the other output doubles, while the torque that is directed through it (50%) remains the same. Basically, that single output takes on a 2:1 leverage over the input, which is why torque is limited to 50%.

 

Not very long ago, that's what they said about monocoque construction vs. body-on-frame construction.

And not very long ago, that's what they said about AWD (i.e. a system with a center diff and non-locking axles) systems vs. manually-locked 4WD systems.

What those people didn't take into account (by being locked into a paradigm of absolutism) is that there are many shades of usage between hardcore off-road vs. purely on-road.

I think your assertion that a RX is designed for exactly the same on-road duty as the RLX is merely a bad assumption. I think to use that to justify that the system differences as purely a function of cost assumes so many other things to be a non-sequitur built upon the initial bad assumption.

 

Half true. You can't beat 50%, but you can make 100% higher.

The torque on an open differential is the same between the two wheels (each gets 50%), up to the limit of adhesion of one of the wheels. If you have one wheel losing traction, the other wheel can have no more torque than the wheel losing traction. The only way to divert more torque to the wheel with traction is to brake the wheel that is spinning thus adding torque to that wheel (from the diff's point of view), and thus to the other side as well.